[cpp-threads] Yet another visibility question

[Peter Dimov]

Alexander Terekhov wrote:
> On 10/9/06, Peter Dimov <pdimov at mmltd.net> wrote:
>> Alexander Terekhov wrote:
>>> On 10/8/06, Peter Dimov <pdimov at mmltd.net> wrote:
>>>> Consider:
>>>>
>>>> // initial values 0
>>>>
>>>> // thread 1
>>>>
>>>> x = 1; // ordinary store
>>>> store_release( y, 1 );
>>>>
>>>> // thread 2
>>>>
>>>> r1 = load_raw( y ); // assume this reads 1
>>>> store_release( y, r1+1 ); // ... and this stores 2
>>>>
>>>> // thread 3
>>>>
>>>> r2 = load_acquire( y ); // assume this reads 2
>>>>
>>>> if( r2 == 2 )
>>>> {
>>>>     assert( x == 1 ); // ordinary load
>>>> }
>>>>
>>>> Will the assert pass?
>>>
>>> Uhmm, it will pass under classic RC model. The store to y in thread
>>> 1 is allowed to be made visible with respect to any other thread
>>> only after the preceding store to x is made globally visible.
>>
>> Yeah. But if the store to x is guaranteed to be made visible only to
>> threads that observe the subsequent store_release to y, T3 is not
>> guaranteed to see it, because it didn't observe T1's store to y, but
>> rather T2's.
>
> Wouldn't that mean that T3 can observe y changing from 2 to 1 (instead
> of 1 to 2) or somehow it would be guaranteed that T3 can never see y
> == 1. I just can't imagine such a model.

It probably can't happen in practice; I'm not sure. It just seemed to me 
that this is the case under the proposed C++MM, and Hans has confirmed it. 
Whether this is good or bad, I don't know, but a refcounting under these 
rules seems to need acq+rel ("ordered") decrement.